However, I'm not sure this is the cause. Without changing any settings, I'm able to snip captures (in colour) from my trading platform, and upload so they display correctly. The problem appears to be affecting Microsoft Excel captures only - is it possible that the underlying software can affect the resolution settings?

Thanks for participating justpi - but it is a WP issue. i can capture and publish on other platforms - just not WP. On investigation, the settings were not changed. However, the problem has now disappeared.